If growing up with two parents is good for children, why not try three?

That’s a question we might have to start asking aftera Suffolk County judge granted “tri-custody” of a 10-year-old boy this month to a Long Island couple and a neighbor with whom they were having a threesome.

Dawn and Michael Marano married in 1994, but in 2001 they invited downstairs neighbor Audria Garcia to, well, join them. Since Dawn was infertile, the three decided that Michael would father a child with Audria but the three of them would raise the children together.

Which worked out fine until Audria and Dawn decided they were happier as a twosome with the child, and left. Then Michael sued for custody.

The judge decided that all three parents were raising their children “in a loving environment” and so granted Dawn Wednesday nights with the boy, as well as one week of vacation during the school year and two weeks in the summer. Audria has residential custody, and Michael gets the boy on the weekends.

As polyamory becomes a more common type of arrangement, more such custody decisions may follow. And now it turns out that medical science seems to be pushing us in this direction.

Last week, Newcastle University in England was granted permission by the British government to make a test-tube baby using the DNA of two different women, effectively giving it three genetic parents.

In Newcastle’s case, the procedure is intended to replace an egg’s defective DNA with healthy DNA from a female donor. Yet it’s easy to see why other parents might consider it.

As more same-sex couples use in-vitro fertilization to have children, it’s not uncommon to find that both parents want a genetic connection to the child — both for legal and emotional reasons.

But before we head down this path, it might be useful to know what the effects are of multiple parents on children. There’s not a lot of research out there on these arrangements and what exists seems hopelessly naïve.

When asked about polyamorous relationships, Maria Pallotta-Chiarolli of Deakin University in Australia told the BBC that “most children are really happy growing up with lots of adults, in fact most kids love it.”

She gushes that “these children are more insightful and wise, and open to understanding diversity and many forms of religion and culture.”

Indeed, there are now clinical psychologists who are decrying “couples privilege”: the presumption that “socially sanctioned pair-bond relationships involving only two people are inherently more important, ‘real’ and valid than other types of intimate, romantic or sexual relationships.”

But “the more the merrier” theory of parenting hasn’t worked out so well in other contexts.

“The research on stepfamilies — where you often have effectively three parents in a child’s life — is clear,” says Brad Wilcox, who leads the National Marriage Project. Kids in more complex families are more likely to struggle.

When there are different adults coordinating with one another, there can be different discipline, schooling and general parenting philosophies involved.

Wilcox explains: “The complexity of three parents in the mix seems to be suboptimal for kids.” When children are caught in parents’ conflicts or they’re the subject of parents’ conflicts, they’re more likely to experience anxiety, behavior problems and difficulty in school.

Robin Fretwell Wilson, who directs the Program in Family Law and Policy at the University of Illinois, agrees that the complexity of the arrangements is a problem. “Kids are not like a pizza you can slice up six different ways.”

She notes that custody arrangements between two people are difficult enough. “How will you do it with three and four?” Wilson, who was adopted as a child and is a divorced mother herself, has a unique perspective on the question of how many parents a child needs.

She notes that in some ways, these questions have been in the courts for years. Couples who use surrogates, for instance, often have a child with two genetic parents and one “birth” parent. States vary in terms of the rights they assign to each.

In some states, the mother who gives birth to the child has precedence over the woman who gave the egg. In other states, it’s the reverse.

In California, it depends on “intention”: When the whole process started, whose child was this supposed to be?

Which brings us back to Dawn and Michael and Audria, who very much intended to be raising children in a group marriage. Says Wilson, “If you start slicing a pizza three ways, the person who had half before now has less.”

You could say that a child’s love is not a fixed pie, but the hours in a day sure are: “It wildly complicates things to have another co-parent or legal parent. If we go too far with this, we are going to create administrative nightmare.”

While Wilson isn’t suggesting more restrictive laws — “we don’t tell people they can’t have three people living in their house” — she can’t help but notice that “at the back end when they don’t work out, the kids pay the price.”

For them, anyway, three is a crowd.

Naomi Schaefer Riley is a senior fellow at the Independent Women’s Forum. Twitter: @NaomiSRiley